The subject matter of the present invention is a spark ignited internal combustion engine modification for improving control of the part throttle spark advance to provide improved fuel economy and driving performance when varying amounts of exhaust gas recycle are introduced into the intake system of the engine.
It is known that a reduction of nitrogen oxide (NO.sub.x) content of exhaust gases can be achieved by recycling part of the exhaust gas from an internal combustion engine back into the intake system as part of the fuel-air charge. This is accomplished by providing a conduit from the exhaust system into the intake system of the engine and providing means for controlling the amount of exhaust gas which is fed into the intake system.
In conventional systems the vacuum advance used on a distributor increases the spark advance as manifold vacuum increases. In these systems, as the load increases, the intake manifold vacuum decreases and the spark advance decreases. Furthermore, when varying amounts of EGR (exhaust gas recycle) are added to the intake mixture the manifold vacuum is lowered accordingly. Thus, in a conventional distributor with a conventional spark advance system the spark advance decreases in response to the reduction in manifold vacuum caused by load increases, addition of EGR or both. However, it is under conditions of EGR introduction that the engine requires an additional ignition advance for optimum performance. Calibrating the distributor to a more advanced setting to compensate for the retarding effect of EGR introduction has the disadvantage of causing the ignition to be over-advanced during warmup or any condition where the EGR is off thereby resulting in knock and possible engine damage.
With the system of the present invention additional spark advance is provided during periods when EGR is used. Under conditions where the EGR is off no additional spark advance is provided and the distributor operates at the initial spark advance mode.